Throughout this application various publications are referred to in parentheses. Full citations for these references may be found at the end of the specification. The disclosures of these publications are hereby incorporated by reference in their entirety into the subject application to more fully describe the art to which the subject invention pertains.
Congenital Heart Disease (CHD) occurs in approximately 9/1,000 live births and 25% of those have critical congenital heart disease (CCHD), defined as requiring surgery or catheter intervention in the first year of life (Mahle, 2009). CCHD can lead to severe complications, including cardiovascular collapse, neurologic injury, and even death if not detected in a timely fashion. Newborn infants with CCHD may have hypoxemia that is not clinically apparent prior to hospital discharge, and not all cases are detected by physical examination (Richmond, 2001).
Pulse oximetry is an objective, non-invasive test that quantifies the percent of hemoglobin that is saturated with oxygen and is widely used as a screening tool for early detection of CCHD (Koppel, 2003; Mahle, 2009; Thangaratinam, 2012). Pulse oximetry screening of all newborns for CCHD has been endorsed by many organizations including the American Academy of Pediatrics (AAP), American College of Cardiology, and American Heart Association (Kemper, 2011; Mahle, 2012). However, the screening as it is currently performed has low sensitivity for coarctation of the aorta (CoA) (Mahle, 2009; Prudhoe, 2013). Coarctation is one of the most common CCHD overall with an incidence of 4/10,000 (Reller, 2008), and the diagnosis of CoA by physical examination is often delayed (Punukollu, 2011).
In patients with CoA, blood flow through the constricted region of the aorta is reduced and the arterial blood pressure and pulse pressure in the upper body are higher than those in the lower body (Beekman, 2001; Goldman, 1949). Previous studies have recommended measurement of upper and lower limb blood pressures to improve the clinical diagnosis of CoA (Beekman, 2001; Brierley, 2002). However, the variation in blood pressures of healthy newborns' extremities is too wide for this method to reliably assess for CoA (Crossland, 2004).
Coarctation may be detected by differential cyanosis on pulse oximetry screening if the test is performed at a point in time when the ductus arteriosus is patent, thereby allowing blood to flow from the right ventricle to the descending aorta. However, in approximately half of all cases of CoA, oximetry demonstrates “normal” saturation, likely because screening is performed after the ductus arteriosus has closed (de Wahl-Granelli, 2007; Mahle, 2009). Despite this “normal” arterial oxygen saturation, the reduction in aortic blood flow causes ischemia to develop in the lower half of the body (Ward, 1990). As a result, the risk of morbidity and mortality rapidly increases in cases of delayed diagnosis of CoA (Punukollu, 2011).
A thorough physical examination includes simultaneous palpation of the radial and femoral pulses. This technique is important for determining the relative pulse volume and whether there is a delay in pulse transmission (MacLeod, 2009). Normally, the radial and femoral pulse upstrokes occur simultaneously. Delayed arrival of the femoral pulse, often associated with diminished amplitude, is indicative of CoA. However, this physical examination technique is challenging to perform in the neonatal population, and a reliable alternative method has not yet been developed. Moreover, the development of these findings commonly occurs after 48 hours of life at a time when many infants have already been discharged home from the hospital (Ward, 1990).
Photoplethysmography (PPG) is the measurement of the systolic increase in tissue blood volume by light transmission through an extremity. The systolic increase in the arterial blood pressure is accompanied by an increase in arterial blood volume resulting in reduced light transmission. PPG wavelengths in the red and infrared regions also serve as the basis for pulse oximetry (Sinha, 2013).
The present invention addresses the need for improved methods and apparatus for detecting aortic coarctation in newborns using photoplethysmographic measurements.